The present invention relates to a method and an apparatus for bonding optical disk substrates, and more particularly, a method and an apparatus for bonding substrates in pairs to be optical disks by setting with light via a photosetting type adhesive, thereby manufacturing bond type optical disks utilized for DVDs (Digital Versatile Disks), etc.
Techniques are known whereby a plurality of substrates having recording layers are bonded each other thereby constituting optical disks for DVDs, so that multilayer recording is achieved.
The Applicant already proposed techniques related to the bond method and apparatus as disclosed in Japanese Patent Application Nos. 8-73536 and 8-89243.
Specifically, according to the proposal, an adhesive of a type set with ultraviolet rays or the like type is injected to a gap of a pair of substrates facing each other via the relatively small gap. The adhesive is spread to the entirety of the gap, whereby a layered body comprising the pair of substrates and the intermediate adhesive layer is obtained. With the projection of ultraviolet rays to the layered body or by the like manner, the adhesive is set and the substrates are bonded to each other.
In order to read stored data on or write data to an optical disk, a laser light is used. The laser light is projected from outside of the optical disk while the optical disk rotates at high speed. The laser light passing through the transparent substrate of the optical disk is irradiated onto a recording layer. At this time, the laser light is required to be correctly shed onto a predetermined position of the recording layer because the optical disk has a considerably high recording density. Meanwhile, it is impossible to focus the laser light correctly on the predetermined position of the recording layer if the recording layer and the whole optical disk show a poor degree of flatness. Thus, information errors can easily occur.
Under the circumstances, a high degree of flatness with no warp or distortion is required to be maintained in the manufacture process of the optical disk.
However, optical disks of the aforementioned bond type, more strictly, disk substrates tend to warp, thereby decreasing the degree of flatness when the adhesive is being set with light.
The reason is the heat generated by an ultraviolet projection lamp used in setting the adhesive. A material of the optical disk is expanded by the heat at the projection of light, then shrunken and returned to an initial state subsequent to cooling, upon completion of the photosetting process. The expansion and shrinkage by the heating and cooling takes place in an unbalanced state between the material at the side close to the lamp and the material at the side away from the lamp, thereby causing the optical disk to warp. The substrate of the optical disk close to the ultraviolet projection lamp has a larger quantity of heat generated thereon because of a larger quantity of light projected thereto and therefore expands greatly, whereas the substrate opposite to the above substrate via the adhesive has a smaller quantity of heat generated thereon and expands less. When the adhesive is completely set in this state with a difference in expansion amount between the substrates, the cooling makes the substrate close to the projection lamp which has expanded more shrink more than the other substrate and consequently the optical disk is deformed to warp towards the projection lamp due to a shrinkage stress.
The deformation to the optical disk is caused by reasons other than the above. For instance, the optical disk is deformed through the expansion and shrinkage process at the time of setting the adhesive, which practically originates from thermal deformation, etc. and an imbalance in material structure brought about in the photosetting process or production process prior to the photosetting process. The deformation presents not only a uniform warp from the center to the outer circumference, but can also cause local unevenness or swell in radial and circumferential directions within a plane.
Temperatures of base materials and apparatuses handling the substrates change with time in a production line where optical disks are produced continuously, sometimes causing an influence on temperature, and the expansion and shrinkage of substrates. Moreover, changes in temperature, and expansion and shrinkage of the substrates are influential to a deformation amount of the optical disk. Particularly for a while when the production line starts to operate after a pause, a temperature condition within the production apparatus is not stable and therefore, the deformation amount of the optical disk is apt to vary.
Environmental conditions where the optical disk is handled also influences the generation of the deformation. For example, a change in humidity influences the expansion and shrinkage. The influence is large when a humidity-absorbing synthetic resin is used as the substrate material.
The object of the present invention is therefore to provide a method and an apparatus for bonding optical disk substrates capable of obtaining optical disks with a good degree of flatness by preventing the optical disks from being deformed, for example, warped, when manufactured through bonding disk substrates.
In order to achieve the aforementioned objective, the present invention provides a method and an apparatus for bonding optical disk substrates, whereby a setting light is projected to a pair of optical disk substrates having a photosetting adhesive layer interposed therebetween, thereby photosetting the adhesive layer and bonding the substrates. According to the present invention, a device is arranged for preventing optical disk substrates from being deformed when the adhesive layer is set with light.
In accomplishing these and other aspects, according to a first aspect of the present invention, there is provided a method for bonding optical disk substrates. The method comprises placing a pair of optical disk substrates on a stage while securing a height difference at an upper face of the stage loading the substrates between an upper face at a side of a center of the substrates and an upper face at a side of an outer circumference of the substrates, and photosetting the adhesive layer on the stage by irradiating a setting light to the substrate pair, thereby photosetting the adhesive layer and bonding the substrates.
According to a second aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, wherein in the photosetting, warp deformation caused when the substrate pair finishes the photosetting operation is offset by deforming the substrate pair tracing the height difference at the upper face of the stage because of a weight of the substrate pair.
According to a third aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, wherein the height difference of the stage is realized by a step whereby the substrates are made higher at the center than at the outer circumference.
According to a fourth aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, wherein the height difference of the stage is realized by a taper, whereby the substrates are lowered from the center to the outer circumference.
According to a fifth aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, the method further comprising controlling temperature of at least a front surface side one of the substrates thereby reducing a temperature difference between both faces of the substrate pair when the adhesive layer is photoset.
According to a sixth aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, the method further comprising making a temperature distribution of the substrates uniform in a plane direction through controlling temperature of the substrates when the adhesive layer is photoset.
According to a seventh aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, wherein in the photosetting, the setting light is irradiated in a step of temporarily setting the adhesive layer with a lower quantity of light than required for completely setting the adhesive layer and a succeeding step of practically setting the adhesive layer with the required quantity of light for completely setting the adhesive layer.
According to an eighth aspect of the present invention, there is provided a method for bonding optical disk substrates according to the first aspect, the method further comprising controlling a humidity of the substrate pair when the adhesive layer is photoset.
According to a ninth aspect of the present invention, there is provided an apparatus for bonding optical disk substrates, the apparatus comprising a stage on which a substrate pair comprising optical disk substrates in a pair with a photosetting adhesive layer placed in between, when the adhesive layer is to be photoset, the stage securing a height difference at an upper face of the stage loading the substrates between an upper face at a side of a center of the substrates and an upper face at a side of an outer circumference of the substrates, and a light source for irradiating a setting light to the substrate pair, thereby photosetting the adhesive layer and bonding the substrates.
According to a 10th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates, according to the ninth aspect, wherein the stage comprises a spacer disposed detachably to the stage at a side of a center of the substrate pair and projecting from a front face of the stage.
According to an 11th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the 10th aspect, wherein a projection amount of the spacer from the front face of the stage is adjustable.
According to a 12th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the ninth aspect, wherein the stage comprises a front face thereof which is tapered to be gradually lower from the center of the front face to an outer circumference of the front face.
According to a 13th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the 9th aspect, the apparatus further comprising a temperature control device for adjusting a surface temperature of the stage.
According to a 14th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the 13th aspect, wherein the temperature control device comprises a temperature control medium passage built in the stage, and a medium feed device for feeding a temperature control medium to the passage.
According to a 15th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the ninth aspect, the apparatus further comprising a shutter part set between the substrate pair and a light source for irradiating the setting light to the substrate pair, for controlling the passing of the setting light.
According to a 16th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the 15th aspect, wherein the shutter part comprises a shielding member for shielding the setting light so as to not pass, a light-passing space set at part of the shielding member to permit the setting light to pass, and a shielding member driving device for selectively arranging the light-passing space at a position corresponding to the substrate pair and a position shifted from the substrate pair.
According to a 17th aspect of the present invention, there is provided an apparatus for bonding optical disk substrates according to the 16th aspect, wherein the shielding member is constituted of a pair of shielding members each having the light-passing space opened at one side end opposite to the counterpart shielding member, and the shielding member driving device has an interlocking reciprocation device for reciprocating the pair of shielding members interlockingly in opposite directions to each other.